Rat kidney WT1 cDNAs contain either a thymidine or a cytosine residue at position 839. Genomic WT1 DNA contains only T839. To explain these results, we propose the WT1 transcript undergoes RNA editing in which U839 is converted to C, resulting in the replacement of leucine 280 in WT1 by proline. RNA editing at the same nucleotide was observed in WT1 cDNAs from human testis. In functional assays, the WT1-leucine280 polypeptide repressed the EGR-1 promoter in in vitro assays approximately 30% more efficiently than WT1-proline. Edited WT1-C839 mRNA was barely detectable in neonatal kidney, whereas adult rat kidneys contained both U839 and C839-WT1 mRNA, suggesting a role for the two protein isoforms in growth and differentiation.